Compliant wirebond pedestal

ABSTRACT

A wire bonder ( 900 ) with a rigid pedestal ( 902 ) having resilient inserts ( 920 ). A package ( 904 ) placed on the pedestal ( 902 ) contains an electrical device ( 906 ). The bond pads on the electrical device ( 906 ) are electrically connected to bond pads on the package ( 904 ) by a series of bond wires ( 908 ) through use of a well know bonding process. A vacuum source holds the package ( 904 ) against the pedestal ( 902 ) deforming the resilient strips ( 920 ) located in the rigid member ( 902 ) of the pedestal and ensuring good contact between the ground pads of the package ( 904 ) and conductive resilient members ( 920 ). The resilient members ( 920 ) are conductive and electrically connect the package grounds to a system ground ( 922 ).

FIELD OF THE INVENTION

This invention relates to the field of semiconductor manufacturing,particular to semiconductor packaging, more particularly to the processof attaching bond wires between a package and a semiconductor devicethat is sensitive to particles generated by the bond out process.

BACKGROUND OF THE INVENTION

Micromechanical devices are small structures typically fabricated on asemiconductor wafer using techniques such as optical lithography,doping, metal sputtering, oxide deposition, and plasma etching whichhave been developed for the fabrication of integrated circuits.

A digital micromirror device (DMD™), sometimes referred to as adeformable micromirror device, is a type of micromechanical device.Other types of micromechanical devices include accelerometers, pressureand flow sensors, gears and motors. While some micromechanical devices,such as pressure sensors, flow sensors, and DMDs have found commercialsuccess, other types have not yet been commercially viable.

Digital micromirror devices are primarily used in optical displaysystems. In display systems, the DMD is a light modulator that usesdigital image data to modulate a beam of light by selectively reflectingportions of the beam of light to a display screen. While analog modes ofoperation are possible, DMDs typically operate in a digital bistablemode of operation and as such are the core of the first true digitalfull-color image projection systems.

Micromirrors have evolved rapidly over the past ten to fifteen years.Early devices used a deformable reflective membrane which, whenelectrostatically attracted to an underlying address electrode, dimpledtoward the address electrode. Schlieren optics illuminated the membraneand created an image from the light scattered by the dimpled portions ofthe membrane. Schlieren systems enabled the membrane devices to formimages, but the images formed were very dim and had low contrast ratios,making them unsuitable for most image display applications.

Later micromirror devices used flaps or diving board-shaped cantileverbeams of silicon or aluminum, coupled with dark-field optics to createimages having improved contrast ratios. Flap and cantilever beam devicestypically used a single metal layer to form the top reflective layer ofthe device. This single metal layer tended to deform over a largeregion, however, which scattered light impinging on the deformedportion. Torsion beam devices use a thin metal layer to form a torsionbeam, which is referred to as a hinge, and a thicker metal layer to forma rigid member, or beam, typically having a mirror-like surface:concentrating the deformation on a relatively small portion of the DMDsurface. The rigid mirror remains flat while the hinges deform,minimizing the amount of light scattered by the device and improving thecontrast ratio of the device.

Recent micromirror configurations, called hidden-hinge designs, furtherimprove the image contrast ratio by fabricating the mirror on a pedestalabove the torsion beams. The elevated mirror covers the torsion beams,torsion beam supports, and a rigid yoke connecting the torsion beams andmirror support, further improving the contrast ratio of images producedby the device.

Micromirror devices have proven very difficult to manufacture. Not onlyare the steps of forming the mirrors difficult to perform in aproduction environment, the completed device is extremely sensitive todebris generated by the production process. While most semiconductordevices can be washed to remove debris and contaminants, the surfacetension of a liquid used to wash the micromirror device destroys themirror array. Therefore, extreme caution must be used to avoid creatingdebris once the mirrors are fully formed and the sacrificial layers onwhich they were formed have been removed.

One process that causes failures is the package bond out process. Oncethe completed device has been attached to the device package, bond wiresare added between bond pads on the integrated circuit and bond pads inthe package. A reliable electrical ground between the package and thebond machine is necessary for the wire bonder to electrically test theconnection between the gold bond wire and the integrated circuit orpackage. A very small electrical current is applied to verify theelectrical connection through a very large resistance path to ground. Itis critical that a good ground is maintained between the package groundand the wire bonder. If the ground fails, the wirebond monitoring system(WBMS) will sense an open circuit between the bond wire and the groundand assume that the failure is due to a poor connection between the bondwire and the bond pad when in fact the bond wire connection may be good.

Prior art mechanisms used a clamp pressed against the seal ring at thetop of the DMD package to ensure an adequate ground. While the clampestablished and maintained a good ground between the wire bonder and thedevice package, it also generated debris particles from contact betweenthe package and the clamp. Since the contact was on the top of thepackage near the device, the particles generated could easilycontaminate the mirror array. What is needed is a method of holding thepackage in place and establishing a reliable ground connection withoutgenerating debris that can enter and damage the mirror array.

SUMMARY OF THE INVENTION

Objects and advantages will be obvious, and will in part appearhereinafter and will be accomplished by the present invention whichprovides a method and system for holding a package in place on awirebond machine while maintaining a reliable ground connection betweenthe machine and the package without creating debris that can enter thepackage and harm the device being packaged.

One embodiment of the claimed invention provides a holder forrestraining and electrically grounding a component during a wire bondingprocess. The holder comprising: an electrical ground, a rigid pedestal,and a conductive resilient member supported by the rigid pedestal andelectrically connected to the electrical ground. The conductiveresilient member operable to engage electrical contacts on the componentelectrically connecting the contacts with the electrical ground.

According to another embodiment of the present invention, a wirebondmachine is provided. The wirebond machine comprising: an electricalground, a source of bond wire, a capillary tube holding a portion of thebond wire, an arm attached to the capillary tube for pressing the wireheld in the tube against bond pads on a device and package, and a rigidpedestal for supporting the device and package. The rigid pedestalcomprises a conductive resilient member electrically connected to theelectrical ground operable to engage electrical contacts on the packageand electrically connect the contacts to the electrical ground.

According to yet another embodiment of the present invention, a methodof attaching bond wires to a semiconductor device and package isprovided. The method comprises the steps of: providing an electricallyground, providing a rigid pedestal having a vacuum cavity and at leastone conductive resilient member on a surface of the rigid pedestal,placing a component on the surface, the component comprised of a packageand an electrical device inside the package, holding the componentagainst the surface using a vacuum applied to the vacuum cavity to forma ground connect between the component and the conductive resilientmember, attaching a bond wire to a bond pad on the package and a bondpad on the electrical device inside the package.

The disclosed invention provides the technical advantage of providing areliable ground while avoiding the introduction of debris that can harmdebris-intolerant devices. The reliable ground is necessary to avoidfalse errors from the bond wire test process. The disclosed pedestal canbe used with a variety of packages reducing tool set-up time andoperation intervention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a plan view of a rigid pedestal according to one embodiment ofthe present invention.

FIG. 2 is a side view of the rigid pedestal of FIG. 1.

FIG. 3 is a front view of the rigid pedestal of FIG. 1.

FIG. 4 is a front view of the rigid pedestal of FIG. 1 showing theaddition of conductive resilient strips to engage and hold a devicepackage.

FIG. 5 is a plan view of a rigid pedestal according to one embodiment ofthe present invention.

FIG. 6 is a side view of the rigid pedestal of FIG. 5.

FIG. 7 is a front view of the rigid pedestal of FIG. 5.

FIG. 8 is a side view of the rigid pedestal of FIG. 5 showing theaddition of conductive resilient strips to engage and hold a devicepackage.

FIG. 9 is a schematic view of a wirebonding machine using the disclosedbonding pedestal to hold a package during the bonding process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A new holding mechanism has been developed that establishes a firmattachment between a package being bonded and the wirebond machine. Themechanism relies on a conductive resilient material to make electricalcontact with ground stations on the package. The resilient material isideally coupled with a rigid pedestal to hold the resilient material andprevent excessive deformation of the resilient material or movement ofthe package being bonded. A vacuum cavity and port is typically providedin the pedestal to allow a vacuum to hold the device against theresilient material and pedestal.

FIG. 9 is a schematic view of a wire bonder 900 using the pedestal 902provided by the present invention. A package 904 placed on the pedestal902 contains an electrical device 906. The bond pads on the electricaldevice 906 are electrically connected to bond pads on the package 904 bya series of bond wires 908. Each bond wire 908 is attached to the bondpads through a process well known in the semiconductor industry. Alength of bond wire 910 is provided and extended through a capillarytube 912. A spark melts the end of the bond wire to form a ball. Thecapillary tube presses the ball against a bond pad on the device 906.The capillary tube 912 vibrates so that the combination of pressure andvibration cause the ball of the bond wire to form an intermetallic bondwith the bond pad of the device 906. Heat is often applied to facilitatethis bond.

After forming the ball bond, the capillary tube pays out bond wire asthe arm 914 of the bonding machine 916 moves the capillary tube and bondwire to a bond pad of the package 904. The bond wire is pressed againstthe package bond pad and vibrated—creating a bond between the wire and abond pad on the package 904. The wire is then sheared off and theprocess repeated for each connection between the device 906 and package904.

During the bonding process, the wire bonder applies an electricalcurrent to the wire to test for continuity between the wire and bondpad. Each bond pad has a high impedance path to one or more packageground pads on the bottom of the package. It is necessary that thesepackage ground pads have a reliable ground connection to complete thetest circuit. If the package is not grounded the wire bonder will detectan open circuit between the wire and the ground connection and assumethat the bond between the bond wire and the bond pad is bad.

As described above, prior art wirebond machines used an overhead clampto press on the top surface of the package. Pressing on the top of thepackage provided good contact between the pads on the bottom of thepackage and the pedestal supporting the package. The overhead clamp,however, generated a lot of debris that could damage the DMD.

FIG. 1 is a plan view of a rigid pedestal 102 according to oneembodiment of the present invention that provides a reliable groundconnection to the package ground pads. The pedestal provides holes 104to mount and align the pedestal to the rest of the wire bond machine. Acentral vacuum cavity 106 in communication with a vacuum port allows avacuum source 918, shown in FIG. 9, to create a low pressure regionunderneath the package to hold the package against the rigid pedestal102. The rigid pedestal 102 typically is 300-series stainless steel, butcould be a ceramic, or some other material. The rigid pedestal 102includes two slot regions 110 designed to receive a conductive resilientmaterial.

FIG. 2 is a side view, and FIG. 3 a front view, of the rigid pedestal ofFIG. 1. FIG. 4 is a front view of the rigid pedestal of FIG. 1 showingthe addition of conductive resilient strips 112 installed in the slotregions shown in FIGS. 1 and 3. The conductive resilient strips engagethe ground pads on the package being bonded and provide a reliableconnection between the ground pads and a ground connection provided bythe bonding machine, typically through the rigid pedestal when aconductive pedestal is used.

The resilient strips 112 are elastic enough to conform to the device asthe vacuum holds the device against the pedestal—thus ensuring a goodground—yet firm enough to prevent excessive movement by the packagebeing bonded. If the package is allowed to move during thehigh-frequency vibration forming the bond, the size and shape of theball of bond wire would not be consistent and the bond formed would notbe as reliable since some of the scrubbing energy used to form the bondwould be dissipated.

The resilient strips 112 typically are an elastomer with a durametervalue low enough to allow the elastomer to conform to the package. Thematerial must be durable enough to maintain its shape an resiliency overlong periods of time during which many devices are processed. Siliconelastomers promise to provide the required resiliency and durability.The elastomer can be made to conduct by impregnating the elastomer witha conductor, typically a metal such as silver (Ag) or copper (Cu).Although DMDs are typically bonded at room temperature, manysemiconductor devices are bonded at an elevated temperature. If bondingis performed at an elevated temperature to improve the integrity of thebond, an elastomer must be selected that is capable of enduring theselected temperature without deterioration or loss of resiliency.

The resilient strips 112 extend above the face 114 of the rigid pedestal102 by approximately 1 mil. Extension beyond the face of the rigidpedestal ensures the resilient strips make good contact with the packagepads. Too much extension would allow movement of the package asdescribed above, and could also lead to an unreliable package height. Itis important that the bond pads be elevated to a point so that thecapillary tube of the bonder is perpendicular to the package during thebonding process. The size, shape, and location of the resilient stripsis chosen to allow the resilient strips to contact the ground pads onthe bottom of the package. The area covered by the resilient strips 112is limited to restrict motion of the package during the bonding process.

FIG. 5 is a plan view of a rigid pedestal 502 according to a secondembodiment of the present invention. FIG. 6 is a side view, and FIG. 7is a front view, of the rigid pedestal 502 of FIG. 5. Like the priorexample, the rigid pedestal 502 of FIG. 5 includes holes 504 foraligning and attaching the pedestal to the wire bonder. The pedestalalso includes a vacuum cavity 506 and vacuum port 508 as well as slots510 designed to receive resilient strips 512, shown installed in FIG. 8.

FIG. 8 shows the relationship between the resilient strips and thecontact pads 514 formed on the bottom of the package 516. As shown inFIG. 8, the resilient strips 512 are designed to contact some, but notall of the contacts 514. Which contact pads must be contacted by theresilient strip is determined by the design of the device and package.The only requirement is to establish a reliable ground for the bondpads. Depending on how the contact pads are connected together, contactbetween only a few of the pads 514—which are not shown to scale in FIG.8—and the resilient strips is sufficient to reliably ground the package.Excessive contact between the resilient strips and the pads on thepackage leads to motion of the package during the bondout process andshould be avoided.

FIG. 9 shows the improved wirebond pedestal 902 in use. A package 904 isheld against the pedestal 902 by application of a vacuum source 918. Thepackage 904 deforms the resilient strips 920 located in the rigid memberof the pedestal ensuring good contact between the ground pads of thepackage 904 and the resilient members 920. The resilient members 920 areconductive and electrically connect the package grounds to a systemground 922. Although shown as strips in FIGS. 4, 8, and 9, the resilientmembers can be any shape that provide sufficient grounding to thepackage and prevents excessive motion during the bonding process.

Thus, although there has been disclosed to this point a particularembodiment for a wire bonder with a compliant wirebond pedestal andmethod therefore etc., it is not intended that such specific referencesbe considered as limitations upon the scope of this invention exceptinsofar as set forth in the following claims. Furthermore, havingdescribed the invention in connection with certain specific embodimentsthereof, it is to be understood that further modifications may nowsuggest themselves to those skilled in the art, it is intended to coverall such modifications as fall within the scope of the appended claims.

1-12. (canceled)
 13. A method of attaching bond wires to a semiconductordevice and package, the method comprising the steps of: providing anelectrically ground; providing a rigid pedestal having a vacuum cavityand at least one conductive resilient member on a surface of said rigidpedestal; placing a component on said surface, said component comprisedof a package and an electrical device inside said package; holding saidcomponent against said surface using a vacuum applied to said vacuumcavity to form a ground connect between said component and saidconductive resilient member; attaching a bond wire to a bond pad on saidpackage and a bond pad on said electrical device inside said package.14. The method of claim 13, said step of providing a rigid pedestalfurther comprising the step of providing a rigid pedestal having atleast one conductive resilient member comprised of an elastomer on asurface of said rigid pedestal.
 15. The method of claim 13, said step ofproviding a rigid pedestal further comprising the step of providing arigid pedestal having at least one conductive resilient member comprisedof a silicone elastomer on a surface of said rigid pedestal.
 16. Themethod of claim 13, said step of providing a rigid pedestal furthercomprising the step of providing a rigid pedestal having at least oneconductive resilient member comprised of a metal impregnated siliconeelastomer on a surface of said rigid pedestal.
 17. The method of claim13, said step of providing a rigid pedestal further comprising the stepof providing a rigid pedestal having at least one conductive resilientmember comprised of a Ag—Cu filled elastomer on a surface of said rigidpedestal.